1. Field of the Invention
The present invention relates to a liquid crystal display and a method for fabricating the same, and more particularly, to a liquid crystal display and a method for fabricating the same, in which the number of masks is reduced to simplify fabricating process steps and improve yield.
2. Description of the Background Art
With the growth of the information economy, displays are used as visual information transmission media. The type of display to develop depends on requirements of low power consumption, thin profile, lightness in weight, and high picture quality. A liquid crystal display (LCD) corresponding to a main device of flat panel displays (FPDs) has performance that can satisfy such requirements and enables mass production. Accordingly, various new products based on the LCD have been fabricated, and the LCD has been widely used as a substitute for a cathode ray tube (CRT).
Generally, the LCD displays desired images by respectively supplying data signals according to image information to liquid crystal cells arranged in a matrix arrangement and controlling light transmittance of the liquid crystal cells.
The LCD mainly employs an active matrix driving mode in which an amorphous silicon thin film transistor (a-Si TFT) is used as a switching element to drive liquid crystals of a pixel portion.
The amorphous silicon thin film transistor has been practically used as a 3″ liquid crystal portable television since 1986 as its concept had been established by LeComber of United Kingdom in 1979. Recently, a large scaled thin film transistor LCD of 50″ or greater has been developed. Particularly, the amorphous silicon thin film transistor has been actively used as it enables low temperature process steps so as to use an insulating substrate of low cost.
However, the amorphous silicon thin film transistor has limitations on use in peripheral circuits that require high speed operation greater than 1 MHz, due to electric mobility of 1 cm2/Vsec, In this respect, studies for simultaneously integrating a pixel portion and a driving circuit portion on a glass substrate using a polycrystalline silicon (poly-Si) thin film transistor are in progress, wherein the polycrystalline silicon thin film transistor has field effect mobility greater than that of the amorphous silicon thin film transistor.
The polycrystalline silicon thin film transistor has been used for a small sized module such as a camcorder since a liquid crystal color television had been developed in 1982. Since the polycrystalline silicon thin film transistor has advantages of low sensitivity and high field effect mobility, a driving circuit can be directly fabricated on the substrate.
Increase of mobility can improve the operational frequency of the driving circuit portion that determines the number of driving pixels. This facilitates fineness of the display. Also, since distortion of a transmission signal can be reduced by decrease of a signal voltage charging time of the pixel portion, it is possible to improve picture quality.
Furthermore, since the polycrystalline silicon thin film transistor can be driven at a voltage less than 10V in comparison with the amorphous silicon thin film transistor having a high driving voltage of 25V, it has an advantage of low power consumption.
Hereinafter, a structure of the LCD will be described in detail with reference to FIG. 1.
FIG. 1 is a plane view illustrating a structure of a general LCD, especially an LCD with a driving circuit integrated on an array substrate.
As illustrated in FIG. 1, the LCD includes a color filter substrate 5, an array substrate 10, and a liquid crystal layer (not illustrated) formed between the color filter substrate 5 and the array substrate 10.
The array substrate 10 includes a pixel portion 35 and a driving circuit portion 30, wherein the pixel portion 35 is an image display area in which unit pixels are arranged in a matrix arrangement, and the driving circuit portion 30 includes a data driving circuit 31 and a gate driving circuit 32, which are arranged along a contour of the pixel portion 35. Although not illustrated, the pixel portion 35 of the array substrate 10 includes a plurality of gate and data lines arranged on the substrate 10 in perpendicular and horizontal directions to define a plurality of pixel regions, thin film transistors formed in portions where the gate lines cross the data lines, and pixel electrodes formed in the pixel regions.
Each of the thin film transistors serves as a switching element that applies or blocks signal voltages to or off the pixel electrodes, and is a kind of field effect transistor (FET) that controls a flow of current using the electric field.
The driving circuit portion 30 of the array substrate 10 is positioned in the contour of the pixel portion 35 of the array substrate 10 more projected than the color filter substrate 5. The data driving circuit 31 is positioned at a long side of the projected array substrate 10 while the gate driving circuit 32 is positioned at a short side of the projected array substrate 10.
At this time, in the data driving circuit 31 and the gate driving circuit 32, a thin film transistor of a complementary metal oxide semiconductor (CMOS) structure serving as an inverter is used to properly output an input signal.
The CMOS is a kind of integrated circuit of a MOS structure used in the thin film transistor of the driving circuit portion that requires high speed signal processing, needs both an n channel thin film transistor and a p channel thin film transistor, and has speed and density characteristics corresponding to an intermediate level of NMOS and PMOS.
The gate driving circuit 32 and the data driving circuit 31 respectively supply scan signals and data signals to the pixel electrodes through the gate and data lines. Since the circuits 32 and 31 are connected with external signal input terminals (not illustrated), they serve to control external signals input through the external signal input terminals and output them to the pixel electrodes.
Furthermore, the pixel portion 35 of the color filter substrate 5 includes color filters (not illustrated) for displaying colors, and common electrodes (not illustrated) serving as opposite electrodes of the pixel electrodes formed in the array substrate 10.
The color filter substrate 5 and the array substrate 10 constructed as above are provided with a cell gap so that they are spaced apart from each other by a spacer (not illustrated). The color filter substrate 5 and the array substrate 10 are bonded to each other by a seal pattern (not illustrated) formed in the contour of the pixel portion 35 to form a unit LCD panel. At this time, the substrates 5 and 10 are bonded to each other by a bonding key formed in the color filter substrate 5 or the array substrate 10.
Since the aforementioned LCD with a driving circuit uses the polycrystalline silicon thin film transistor, it has advantages of excellent device characteristics, excellent picture quality, fineness, and low power consumption.
However, since the LCD with a driving circuit should be provided with the n channel thin film transistor and the p channel thin film transistor formed on a single substrate, its fabricating process steps are more complicated than those of the amorphous silicon thin film transistor that forms a single type channel only.
In fabrication of the array substrate including the thin film transistor, a photolithographic process is required several times throughout the manufacturing process.
The photolithographic process includes a series of process steps of forming a desired pattern by transferring a pattern printed on a mask onto a substrate on which a thin film is deposited, wherein the series of process steps include coating, exposing and developing process steps of a photoresist. In this case, problems occur in that the photolithographic process reduces production yield and increases the probability of a thin film transistor having a defect.
Particularly, since the mask designed to form the pattern is expensive, the fabricating cost of the LCD increases proportionally if the number of masks for the process steps increases.